Hi Jens,
sourceforge.net/p/ngsolve/ml-ngs/ci/master/tree/
is a small project explaining how to work with NGSolve in C++.
As far as I know there is no similar thing for Netgen but using the files there should make it possible to build a cmake project linking to NGSolve and using the functionality. Linking to Netgen should work similar.
At the usermeeting I've talked about this, the slides are available at
ngsolve.org/user-meeting2017/talks/lackner_ngsc++.pdf
I don't know how helpful they are alone, but probably better than nothing
Best
Christopher